Expression used by cyclists to describe excercise induced low blood sugar levels; being a feeling of light-headedness and weakness in all limbs. Similar to 'The Wall' in running. Has fallen out of usage in recent years due to alternative meanings.

I am feeling a bit light-headed, if I don't have a banana, I think I'm going to bonk.
I've got the bonk.
I bonked really badly and had to get a lift home.

1. Have sexual intercourse with.
2. To hit, particularly hitting a person on the head.
3. During the hayday of MUDs (Multi-User Dungeon games, text-only roleplay like adventure games played mostly over the net) a commonly said or done action as a sign of mild disapproval or for no particular reason. Polite convention require(d/s) the bonked individual respond by saying oif. Some believe not doing so will upset the bonk/oif balance in the universe, with great consequences. Not saying "oif" is somewhat akin to a ( without a ) or unmatched quotation mark. The practice is still somewhat alive on IRC and (to an even lesser degree) other means of net chat. It has, however, to a large degree been replaced by the fish slap.